rpc: preparations for recursion fix
    
    Made several improvements to coding style:
     - same use of KerberosError throughout the module
     - removed some unused variables
     - moved code from try-except blocks if it didn't have to be there
     - preparations for putting most of RPCClient.create_connection()
       to loop
    
    https://pagure.io/freeipa/issue/6796
    
    Reviewed-By: Florence Blanc-Renaud <frenaud@redhat.com>